The present invention relates to exhaust gas sensor operating systems and more particularly to an operating temperature detection system for detecting when the operating temperature of the gas sensor is within its normal operating range.
In a closed-loop controlled combustion engine wherein an exhaust gas sensor is positioned in the exhaust system of the engine for generating an output signal for controlling the air-fuel ratio of the mixture supplied to the engine, the sensor must be above a certain temperature in order to operate properly. If the gas sensor is below this temperature, the sensor operation is abnormal resulting in the internal combustion engine operating unsatisfactorily.
When the exhaust gas sensor is operating below its operating temperature during a prolonged idle condition, its internal impedance will become very high and its maximum voltage will decrease, and its minimum voltage, which is usually zero potential, will rise to the decreasing maximum peak values, so that the gas sensor output fluctuates within a decreasing range of values. Copending United States Application No. 715,652 filed on Aug. 19, 1976 assigned to the same assignee as the present application, discloses a system which comprises negative and positive peak detectors for generating a signal of which the magnitude takes on an intermediate value between the varying maximum and minimum peak values of the exhaust gas sensor. The varying intermediate voltage is used as a reference with which the gas sensor output is compared to generate a deviation signal which indicates the deviation of air-fuel ratio within the exhaust system from a near stoichiometry at which catalytic conversion efficiency is maximized.
However, the operating performance of the oxygen sensor varies between different sensors and certain types of oxygen sensor may exhibit a specific characteristic in which the minimum peak values of the sensor also tend to rise when the sensor temperature falls while the maximum peak values decrease, with the resultant narrowing of the range of voltage fluctuations. Although the prior art approach accomplishes prolongation of closed loop operation when the exhaust gas sensor temperature falls during prolonged idle condition, the threshold value will take on an indefinite value within the narrow range so that the closed loop operation will become unsatisfactory. An object of the invention is to prevent meaningless prolongation of closed loop control operation when the exhaust gas sensor operates below its normal operating temperature during prolonged idle condition so as to minimize the emission of noxious waste products.